1. Technical Field
The disclosure relates to a low-pass filter, comprising an integrator whose unity frequency is adjustable.
2. Description of the Related Art
Low-pass filters are elements that are used in numerous applications, in particular in signal processing subsystems.
The applications that use low-pass filters include the telecommunication reception subsystems, regardless of the standard used, of GSM (Global System for Mobile Communications), W-CDMA (Wideband Code Division Multiple Access), WIFI (Wireless Fidelity) or even WiMax (Worldwide Interoperability for Microwave Access) type, or else also the filtering subsystems used to filter the data read by the hard disk read heads.
Given the increasingly large number of possible applications and the proliferation of the standards, it is desirable for the signal processing subsystems not only to exhibit a significant linearity but also to be able to operate over wide frequency ranges.
In the case of a GSM reception subsystem, the cut-off frequency for the entire reception subsystem is well determined. However, in practice, the cut-off frequency of a GSM reception subsystem can vary within a ratio of 1 to 2 about the mean cut-off frequency. Thus, there is a need for a reception subsystem with a cut-off frequency that can be adjusted according to the variations of the cut-off frequency.
There is also a need for reception subsystems to be reconfigurable. As an example, a reception subsystem initially configured for the GSM standard may be reconfigured for the W-CDMA standard. The cut-off frequency of the low-pass filters that form such reception subsystems should be able to be adapted according to the standard for which the subsystem is reconfigured. In practice, the cut-off frequencies of the various standards may be very different, within a ratio that can range in practice from 1 to 5, or even from 1 to 10.
The read signals from the hard disk read heads also exhibit wide cut-off frequency variations. In fact, in practice, since these frequency variations are linked to the rotation variations of the hard disk and of the position of the read head, they can vary within a ratio from 1 to 20 or even from 1 to 25.
There is therefore a need for reception subsystems that are adjustable or that can be adjusted in order to be able to be used for different applications.
In the prior art, there are low-pass filters of the “GmC” filter type, and low-pass filters of the “active RC” type.
One example of low-pass filter of the first order of the “GmC” filter type 10 is represented in FIG. 1.
The low-pass filter 10 represented in FIG. 1 comprises two types of components, first 12 and second 16 transconductors and a capacitor 14.
The inverting input of the first transconductor 12 is linked to ground. The non-inverting input of this first transconductor 12 is connected to a branch to which is applied the input voltage Vin of the low-pass filter 10. The output of the first transconductor 12 is connected to the inverting input of the second transconductor 16. Said branch linking the output of the first transconductor 12 and the inverting input of the second transconductor 16 is linked to ground via the capacitor 14. The non-inverting input of the second transconductor 16 is linked to ground. The second transconductor 16 comprises a feedback loop between its output and its inverting input. The output voltage Vout of the low-pass filter 10 is measured at the output of the second transconductor 16.
In the case of the first order filter as represented in FIG. 1, the cut-off frequency fc of the filter can be expressed:
      fc    =          gm              2        ⁢        π        ⁢                                  ⁢        C              ,
with gin being the conductance of the transconductors and C being the value of the capacitor 14.
The benefit of such a “GmC” filter lies in the fact that the cut-off frequency can be adjusted by varying the conductance “gin”. Numerous variants of “GmC”-type first order low-pass filters are known to those skilled in the art.
However, such low-pass filters exhibit the drawback of having their linearity depend directly on the linearity of the transconductors 12 and 16. The linearity of the transconductors 12 and 16 cannot easily be improved with the usual techniques.
One example of “active RC”-type first order low-pass filters is represented in FIG. 2.
The low-pass filter 18 represented in FIG. 2 comprises an operational amplifier 20, first 22 and second 24 resistors and a capacitor 26.
Said operational amplifier 20 comprises first and second inputs, an output and a feedback loop between said output and said first input. Said first input is connected to a branch comprising the first resistance 22 to which is applied the input voltage Vin of the low-pass filter 18.
The feedback loop comprises the second resistor 24 mounted in parallel with the capacitor 26. The second input of the operational amplifier 20 is linked to ground.
The output voltage Vout of the low-pass filter 18 is measured at the output of the operational amplifier 20.
The low-pass filter 18 represented in FIG. 2 is a first-order filter, the cut-off frequency fc of said low-pass filter 18 is expressed:
      fc    =          1              2        ⁢        π        ⁢                                  ⁢                  R          2                ⁢        C              ,
with R2 being the value of the second resistor 24 and C being the value of the capacitor 14.
The benefit of such an “active RC” filter is its very good linearity performance. In practice, the high gain of the operational amplifier 20 makes it possible to have a voltage ε between the inputs of the operational amplifier 20 that is very low, ideally zero. The input current of the operational amplifier 20 is also zero, so the output voltage Vout of the low-pass filter 18 will be primarily determined by the first and second resistors 22, 24 and the capacitor 26. Since these components are highly linear, the linearity performance characteristics of an “active RC” low-pass filter are very good.
The drawback with this type of low-pass filter lies in the fact that the cut-off frequency cannot be adjusted other than by modifying the value of the capacitor 16, which is done discretely by connecting or disconnecting other capacitors in parallel with the initial capacitor. This adjustment method increases the complexity of the assembly since it uses multiple additional capacitors and switches to be put in place.